1. Field of the Invention
This invention relates to a variable magnification observation apparatus having an illuminating optical system using an optical fiber.
2. Description of the Prior Art
Generally, in Kohler illumination, it is desirable that the light source be a surface light source. Therefore, if the image of the light source is formed on one end surface of a fiber, the other end surface of the fiber can be made into a secondary surface light source.
The typical optical systems thereof are shown in FIGS. 1 and 2 of the accompanying drawings. As shown, a field stop 3 and an object surface 7 are conjugate with respect to a collector lens 4 and a condenser lens 6, and an aperture stop 2 and the entrance pupil plane 5 of the condenser lens 6 are conjugate with respect to the collector lens 4. Generally, in a low magnification optical system, it is necessary to make the aperture stop small and make the field stop large, and in a high magnification optical system, vice versa. However, the light ray emerging from an optical fiber 1 does not have a uniform angle distribution but has directionality and therefore, as the angle between the end surface 1a of the optical fiber and the emergent light ray becomes deviated from the vertical to a certain degree or more, the quantity of light becomes sharply weaker. In the low magnification illuminating system of FIG. 1, due to such a characteristic of the fiber, the light ray 8 arriving at the marginal portion of the field stop 3 from the center of the fiber is weak as compared with the light ray 9 arriving at the center of the field stop, and where, as an extreme case, the angle of the light ray 8 is greater than the angular aperture (N.A.) of the light beam of the fiber 1, there is no light arriving at the marginal portion of the field stop and the marginal portion of the field of view becomes dark. That is, there is a disadvantage that illumination irregularity occurs. In the high magnification illuminating system of FIG. 2, illumination irregularity does not occur, but the light beam 10 of a great angle of emergence indicated by hatching is weak as compared with the light beams 11 and 11 emerging from the fiber end surface 1a perpendicularly thereto, and this has led to a disadvantage that the illuminating light cannot be efficiently utilized.